LAB SCALE ANFD (AGITATED NUTSCHE FILTER DRYER):
Just like a Commercial Scale ANFD, the Lab Scale ANFD too is popular in Pharmaceutical and Chemical Laboratories and/or R& D Units. Many a times the API Or chemical substances under R&D are highly expensive. Taking trial batches with such expensive material on a commercial scale is too risky and expensive. Hence, the Lab Scale Units Or Pilot Plant units of such equipments becomes extremely convenient and economic.
WORKING PRINCPLE OF LAB SCALE ANFD:
The working principal of the Lab Scale ANFD is same as that of the commercial class ANFD, where the moisture Or solvent is removed by a series of Unit Operations viz. Filtration and Vacuum Drying.
Lab Model too consists of a small size jacketed/ limpeted vertical cylindrical stirred tank vessel (somewhat looking like a pressure vessel), which has its shell divided into two halves viz. Main Shell and Bottom Shell. A small mesh Or sieve separates the two halves. A filter cloth (mostly PP or some other membrane like material) covers the sieve. The excess water Or solvent from the wet mass is filtered through the mesh and is collected in the bottom shell, whereas, the remaining part of the water Or solvent is removed by evaporating it (through a heating action) and vacuum drying the wet mass.
In short, the solvent Or water to be removed, is separated and collected at two different places viz.
1. Bottom Shell: through mechanical filtration facilitated by:
(a) Air/ Inert Gas Pressure in the main shell Or
(b) Agitation created by the vertically adjustable specially designed Agitator Or
(c) Vacuum created in the bottom shell (which pulls the excess water from the mass towards the bottom shell).
2. Vacuum Trap Or Receiver: by means of condensation of solvent/water vapors generated inside the main shell.
MAJOR COMPONENTS OF A LAB SCALE ANFD:
ANFD is basically is an integrated system consisting of various components as listed below:
1. Main Shell: This small size vertical cylindrical and Jacketed/ Limpeted vessel is usually constructed larger in diameter and smaller in height. This is so because bigger is the diameter of the vessel, more is the filtration area, and hence, faster is the filtration. The main shell is further insulated (and also cladded) with hot insulation material such as Mineral Wool Or Rock Wool.
2. Agitator Or Vertical Stirrer: An agitator is a low RPM (rotations per minute) stirrer, which is controlled by a Stepper Motor for precise movements of the agitator. Just like the commercial scale ANFD, this agitator too is designed to rotate both clockwise, as well as, anticlockwise. Either a Balls-Screw type arrangement Or a hydraulic cylinder is used to manage the vertical back and forth movement of the agitator. Agitator plays an extremely important role of sqeezing the wet cake, mother liquor wash, disturbing the cake and in overall filtration and drying.
3. Perforated Plate with Filter Cloth: The Main Shell and Bottom Shell are concentric and has the same diameter. A perforated mesh Or metal plate along with the filter cloth is sandwiched between the bottom flange of the main shell and top flange of the bottom shell. Basically this creates a filtration barrier between the two vessels.
4. Bottom Shell: The Bottom Shell is meant to collect the filtrate generated our of squeezing action of the cake. This squeezing action can be either because of the pressure generated by the agitator blades, Or because of the air/ neutral gas pressure generated above the cake surface Or seldom because of the vacuumized pull generated from the bottom shell i.e. from the bottom of the filter cloth.
5. Top Dish with Motor, Gear Box, Mechanical Seal, and Hydraulic Cylinders/ Ball Screw action: The most difficult and challenging part in the entire assembly of Lab Scale ANFD is to design the Top Dish. It is so because, it has to have space allocation for multiple nozzles and opening including agitator shaft and mechanical seal, feeding nozzle, handhole (if separate), vacuum nozzle, light glass, sight glass, instrumentation nozzles like Compound Gauge, Temperature Guage (if demanded by client) etc.
Therefore, this torrispherical dish is designed to handle pressure, vacuum and the weight of the complete drive assembly. A single-dry mechanical seal is used to arrest vacuum Or pressure inside the main shell and top dish. Good quality gasket (either food grade silicon Or Viton Or at times PTFE) is used between the flanges of the top dish and main shell. Since it is a small assembly, even a ball-screw kind of an arrangement can be preferable over a hydraulic cylinder for vertical movement of the agitator. Motor is programmable (stepper motor) which ensure highly precise movements.
6. Cake Discharge: Since the entire assembly is small, the dry cake can be manually taken off the main shell by dismantling it and tilting it easily.
7. Condenser: A Shell and Tube Type Heat Exchanger (mostly Vertical one) is used to condense the hot vapors generated inside of the main shell. Usually the process side fluid (vapors) are passed through tubes, whereas, on the Shell side the utility fluid (mostly chilled water Or cooling water) is circulated. However, it can be the other way around too (depending on the heat load calculations). Condenser can be insulated and cladded for avoiding heat loss to the ambience.
8. Receiver: Ideally a receiver is used to collect the condensate from the condenser. However, in many cases, it is also used to collect the filtrate from the bottom shell of an ANFD. In such situations, the Receiver is arranged either below the bottom shell of ANFD Or below the STHE type Condenser, and a pipe manifold with check valves is provided for ensuring both condensate and filtrate can be collected in the receiver. A vacuum breaking arrangement needs to be provided with a Control/ On-Off valve on the receiver, so that the condensate can be feeded to the receiver without breaking the vacuum.
9. Common Skid: A metal skid is provided to support the entire assembly including the ANFD, assembly for Lifting and Lowering of the Agitator, Condenser, Receiver, Vacuum pump, Water Heating system (if available) and Operating Panel.
10. Vacuum Pump: A suitable capacity Vacuum Pump is connected to the system (mostly to the receiver), which creates vacuum inside the ANFD, and, also facilitates the extraction of the vapors, hence complements the condensation process.
10. Water Heating system Or Low Pressure Steam (for heating the ANFD main shell Limpet): Incase of a Hot Water System, a separate vertical cylindrical vessel is provided with external heating i.e. either with steam (LPS) Or with Electrical Heaters. A circulation pump pushes the hot water into the Limpet/Jacket of the ANFD Main Shell, and the return water goes back into the Hot Water Generation tank. The level guage/switch on this tank ensure water doesn't spill over, and, make up water valves gets ON when level goes down. The Temperature sensor on the Hot Waters Tank, as well as, sensor on the Inlet of the Limpet coil together controls the flow of Steam (through PID control valve) Or the Electrical Heater, thus ensures the inlet water temperature stays in control.
Incase of Direct Steam (LPS) Heating, the PID valve ensures steam is throttled and uniform temperature is maintained in the limpet/ Jacket of the ANFD's Main Shell.,
11. Nitrogen Purging system/ Air Pressure system: A nozzle can be provided with control valve, that can facilitate the intermediate and periodic gushing or N2 gas, which pushes the vapors out towards the condenser. Same nozzle can be used for providing the Air Pressure to facilitate the filtration operation. Its important to note here that the Vacuum needs to be stopped temporarily while purging.
TYPICAL SEQUENCE OF OPERATIONS IN ANFD:
A general sequence of operation can be described as per below. It is to be noted that the number of steps, sequence and other protocol can very from company to company and based on the product and most importantly the SOP.
STEP -1 FILTRATION: The wet mass before drying has to undergo filtration first. In this step, either Air Or Nitrogen pressure is applied to the main chamber Or Vacuum is applied to the bottom shell Or both of these done. The excess water or solvent it thus pushed and pulled to pass through the filter cloth. Heating may Or may not be needed in this case. However, the rotating agitator helps in pushing the material down towards the filter media.
STEP -2 SOLVENT/ WATER/ MOTHER LIQUOR WASH: Sometimes the process demands Process Water/ Solvent wash Or Mother Liquor wash to the wet cake. This action (Filtration - ML Wash- Filtration) may be repeated a number of times depending on the process. The agitator is pulled up with the hydraulic cylinder and after adding water/ML, it is taken down with opposite direction rotation which disturbs the settled cake and facilitate proper washing of the cake.
STEP -3 HEATING AND VACUUM DRYING: When the cake is well filtered (or filtered as per SOP), heating is carried under vacuum by circulating the hot utility in the limpet of the main shell. As per the process requirement, as well as SOP, the drying cycle can be repeated Or extended based on the in batch sampling results.
STEP -4 SOLVENT OR ML (MOTHER LIQUOR) COLLECTION: As mentioned earlier, the solvent or ML can be collected in both bottom shell, as well as, in the receiver.
STEP -5 DISCHARGING OF THE DRIED CAKE: When the cake is dried upto expectations, the vacuum is broken and the cake is manually taken out (since it's a small size assembly, hence side discharge valve is not needed).
MATERIAL OF CONSTRUCTION:
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